1. Field
Polymer processing and more particularly to the formation of polymer products used in a variety of applications.
2. Background
Polymer constructs with a balance of porosity, strength, flexibility and chemical inertness or biocompatibility are desired in many biomedical and industrial applications.
In medical implant fields, polymers such as Dacron polyester and expanded polytetrafluoroethylene (ePFTE) have been used for medium and large diameter vascular prosthesis. Dacron prosthesis are generally woven or knitted into tubular constructs. The relatively large pore size resulting from knitting and weaving techniques allows blood to pass through these pores, necessitating either pre-clotting these constructs with the patient""s blood before implantation, or impregnating the constructs with a biocompatible filler. The porosity of ePTFE can be tailored by adjusting the node and fibril structure, and consequently the porosity and pore size, such that blood is contained within the tubular structure under physiological conditions. Neither Dacron, nor ePTFE tubular constructs has however functioned effectively as small diameter vascular prostheses due to problems of thrombosis and anastomotic hyperplasia.
The flexibility, strength, biostability and ability to adjust porosity has also led to ePTFE being used for tissue augmentation in plastic surgery, in dura mater repair in neurosurgery, and for breathable, moisture-barrier cast liners. The combination of flexibility, lubricity and strength have also led to ePTFE use in dental floss.
In the medical device industry, angioplasty balloons are typically formed from thermoplastic nylons, polyesters, and segmented polyurethanes. To reduce the effective profile of the device for ease of delivery into the vasculature, balloons are folded on to the catheters. Upon inflation in the vasculature, the balloons unfold to assume a cylindrical profile. This unfolding generates non-uniform stresses in lesions during inflation. Furthermore, when stents are mounted on folded balloons, their deployment in the vasculature may be non-uniform due to the unfolding process. There is consequently a need for a balloon that is flexible, yet strong with the ability to be delivered in the vasculature in a small tubular profile without folding. Materials with node and fibril structures, that can be rendered auxetic, i.e., having a negative Poisson""s ratio, with appropriate processing are particularly suitable for this application.
In the field of local drug delivery, there is a need for chemically inert and biocompatible microporous drug reservoirs for releasing drugs from transdermal patches. Polymers such as ultra-high molecular weight polyethylene (UHMWPE) may serve this need if they are rendered porous.
In the textile industry, ePTFE barrier layers are used for apparel that needs to be breathable, while preventing moisture from passing through the apparel.
UHMWPE is used as a separator membrane for electrochemical cells such as lithium-ion batteries, supercapacitors and fuel cells. For these applications, microporous UHMWPE membranes provide the right balance of porosity, wettability, flexibility and strength.
U.S. Pat. No. 5,643,511 discloses a process for the preparation of microporous UHMWPE by solvent evaporation from a gel-formed film. The films are stretched uniaxially or biaxially either during solvent evaporation or after solvent evaporation, to achieve the desired porosity. The microporous films thus obtained do not have a node and fibril structure.
U.S. Pat. No. 4,655,769 describes a process for preparing microporous UHMWPE by forming a pseudo-gel of UHMWPE sheet in a solvent, extracting the solvent with a more volatile solvent, evaporating the volatile solvent to create a semi-crystalline morphology and stretching the dry sheet. These films do not exhibit a well-defined node and fibril structure.
In regards to the above applications and limitations of current materials, there remains a desire for porous and flexible polymer constructs having high strength, good chemical inertness and biocompatibility, and which can preferably be made to exhibit auxetic behavior.
A method is disclosed. The method includes, in one embodiment, forming a pseudo-gel of a semi-crystalline polymer material and a solvent. The pseudo-gel is shaped into a first form and stretched. A portion of the solvent is removed to create a second form, and the second form is stretched into a microstructure including nodes interconnected by fibrils. Such polymer article may be used in a variety of applications including, but not limited to, medical device applications such as in catheter balloons, and various grafts. Other applications include, but are not limited to, use in dental floss, sutures, filters, membranes, drug delivery patches, and clothing.
Ultra-high molecular weight polyethylene is one example of a suitable semi-crystalline polymer material. In another embodiment, a method including forming a first form of a pseudo-gel comprising an ultra-high molecular weight polyethylene material and a solvent at a temperature above a crystalline melting point of the ultra-high molecular weight polyethylene is disclosed. The first form is stretched and the solvent is removed to form a second form. The second form is stretched into a microstructure including nodes interconnected by fibrils.
An apparatus is also disclosed. In one embodiment, the apparatus includes a body portion formed of a dimension suitable for a medical device or other application. The body portion includes a polyolefin polymer material including a node and fibril microstructure formed by successive stretching of the polymer material in the presence of a solvent and in the absence of a solvent. In another embodiment, an apparatus including a body portion including an ultra-high molecular weight polyethylene material including a node and fibril microstructure is disclosed.